1. Field of the Invention
The present invention relates to the production of finely divided and free-flowing powders from thermotropic polymers which are capable of forming anisotropic melts, and, more especially, to the production of such powders by grinding granulates of thermotropic polymers via an extrusion/granulation technique characterized by unique parameters of the extrusion operation.
The present invention also relates to the granulates that serve as intermediates in the above technique and which are converted into the final product powders when ground.
This invention also relates to such finely divided and free-flowing powders, per se.
2. Description of the Prior Art
For about the last ten years, there has been an increasing interest in polymers which are capable of forming anisotropic melts, and also in polymers which are capable of forming liquid crystal phases (or organized mesophase) in the melt.
These polymers have different phase transition temperatures which are, in an increasing order of temperatures:
(a) the crystallization temperature on cooling (T.sub.c);
(b) the melting temperature (T.sub.M) at which the liquid crystal phase appears; and
(c) the clearing temperature (T.sub.cl) beyond which the mesophase becomes isotropic.
These temperatures T.sub.c, T.sub.M and T.sub.cl are determined by differential calorimetric analysis (DSC), using a Du Pont apparatus comprising a DSC 910 module coupled with a 9900 central processing unit, on samples subjected to both increasing and decreasing temperature changes of 20.degree. C./minute. The crystallization and melting temperatures correspond to the peaks of the crystallization exotherm on cooling and of the melting endotherm. The nature of the phases which are present is identified by observations using a polarizing microscope equipped with a heated stage. The temperature span of the anisotropy range, an interval which can be represented by the difference T.sub.cl -T.sub.M and which has a value ranging from about ten degrees to several tens of degrees will depend essentially on the structure of the thermotropic polymer.
The above increasing interest in the thermotropic polymers is related to the fact that, in the melt, these polymers spontaneously have a specific orientation and a relatively high degree of organization. This orientation and organization are also present in the articles shaped from these polymers, which impart thereto improved physicochemical and mechanical properties which are not observed in shaped articles which are identical but isotropic. However, given this spontaneous orientation and organization of the material in the melt, the conversion of these thermotropic polymers by extrusion:
(i) when involving the manufacture of shaped articles other than filaments and fibers and consisting of profiles, such as, for example, rods, bars and tubes,
(ii) such extrusion being conducted according to the usual operating technique known to this art for extruding nonthermotropic polymers, in which the temperature of the polymeric material which is selected, both within the cavity of the extruder and in the calibrating die, is situated at a value above the melting temperature of the polymer (in the case of a thermotropic polymer, this value is generally situated within the anisotropy range), results in shaped objects whose fine structure, even after cooling, is highly fibrillar due to the essentially uniaxial orientation of the polymer chains in the direction of flow of the material; as a result, such extruded shaped articles, which display transverse embrittlement leading to delamination phenomena, are not suitable for industrial applications.
When, for example, it is a rod that is recovered at the die outlet, it has been found that it is still impossible, or else very difficult, to obtain:
(i) when a granulation operation is carried out according to the conventional technique for granulating a rod of nonthermotropic polymer, compact granulates of nonfibrillar fine structure that are required for an effective downstream grinding operation;
(ii) when these granules are ground according to conventional technique for grinding granulates based on nonthermotropic polymer, finely divided and free-flowing powders which are suited for such applications as, for example, electrostatic or fluidized-bed coating, or the production of thermoplastic composites by compression molding. The powders based on thermotropic polymer which are produced are particles which are not spherical but are acicular in shape and these powders form a wad (or entanglement) of microfibrils which is not free-flowing, or a mass of fibers of very different sizes which is also not free-flowing.